At present, communications between airborne vehicles and ground stations such as, for example, between airplanes and air traffic control centers, is generally provided over analog wireless very high frequency (VHF) broadcasts using 25 KHz bandwidth assignments (25 KHz analog channels). Public safety communication systems such as, for example, systems employed by emergency responders (e.g., police, fire, and emergency medical technicians) and their dispatchers also often employ similar analog communication channels over a variety of frequencies. Given the large number of analog channels assigned for such uses, a substantial amount of spectrum is occupied and underutilized.
Digital communication systems can provide enhanced capabilities as compared with existing analog air-to-ground/ground-to-air and public safety communication systems such as, for example, an ability to accommodate more system users and improved transmission clarity and enhanced information transmission capabilities. Thus, there may be a desire to replace such analog systems with digital systems, but due to associated costs and other constraints it may not be practical to promptly end the use of such analog communication systems and require that digital systems immediately be employed by all users within a geographic region (e.g., a national or international air traffic control corridor, an air traffic control area surrounding an airport, a county, or a city). Thus, an extensive transition period may be required wherein analog and digital communication capabilities coexist within a given geographic region while users migrate from analog to digital systems. However, identifying a sufficient block of available spectrum for such new digital communication systems may be difficult given the crowded nature of the spectrum, competing uses for available spectrum (e.g., cell phone operations, wireless networks, military applications), and the existence of such a large amount of spectrum that is already assigned to such users.